Modeling the performance of the anaerobic phased solids digester system for biogas energy production

A process model was developed to predict the mass and energy balance for a full-scale (115 t d−1) high-solids anaerobic digester using research data from lab and pilot scale (1–3000 kg d−1 wet waste) systems. Costs and revenues were estimated in consultation with industry partners and the 20-year project cash flow, net present worth (NPW), simple payback, internal rate of return, and revenue requirements were calculated. The NPW was used to compare scenarios in order to determine the financial viability of using a generator for heat and electricity or a pressure swing adsorption unit for converting biogas to compressed natural gas (CNG). ll-scale digester consisted of five 786 m3 reactors (one biogasification reactor and four hydrolysis reactors) treating a 50:50 mix (volatile solids basis) of food and green waste, of which 17% became biogas, 32% residual solids, and 51% wastewater. The NPW of the projects were similar whether producing electricity or CNG, as long as the parasitic energy demand was satisfied with the biogas produced. When producing electricity only, the power output was 1.2 MW, 7% of which was consumed parasitically. When producing CNG, the system produced 2 hm3 y−1 natural gas after converting 22% of the biogas to heat and electricity which supplied the parasitic energy demand. The digester system was financially viable whether producing electricity or CNG for discount rates of up to 13% y−1 without considering debt (all capital was considered equity), heat sales, feed-in tariffs or tax credits.